Rigid polyurethane foams are produced in large amounts by known methods and from known raw materials. A comprehensive presentation of the raw materials used and the applicable methods may be found in Ullmann's "Enzyklopadie der technischen Chemie" (Encyclopedia of Chemical Engineering), 1980, volume 19, pages 301 to 341, and in the Kunststoff-Handbuch (Plastics Handbook), volume VII, "Polyurethanes" by R. Vieweg and A. Hochtlen, Carl Hanser Verlag, Munich, 1966, pages 504 to 544, as well as in "Polyurethane" Kunststoff-Handbuch ("Polyurethanes" Plastics Handbook) 7 by G. Oertel, Carl Hanser Verlag, Munich, 1983, pages 246 to 331.
Rigid polyurethane foams are preferably used for thermal insulation. For this application, they can be foamed into molded articles such as panels. It is also possible to fill hollow spaces of all types with foam, in which case the foam in the hollow space, as a structural element, frequently must also have load-bearing properties. It is furthermore possible to spray rigid polyurethane foams on tabular supports.
The quality and the properties of the rigid foam formed depend to a large extent on the structure and chemical composition of the foam stabilizer used.
As foam stabilizer, polysiloxane-polyoxyalkylene block copolymers are generally used. These stabilizers have a high effectiveness and can be adapted to the foaming system and the foaming method by selecting a suitable structure and composition. However, a disadvantage of these organosilicon stabilizers is their high prices. It has also been observed that the surfaces of the foamed material obtained by using organosilicon stabilizers are difficult to wet. As a result, subsequent protective treatments for the surfaces, such as painting or varnishing, become difficult. Attempts have therefore already been made to develop silicon-free stabilizers, which do not have these disadvantages.
German Patent 2,244,350 describes a method for the preparation of a polyurethane foam by the reaction of a polyisocyanate having at least 2 NCO groups, with a polyether polyol in the presence of a catalyst, a blowing agent and 0.5 to 3 parts by weight, based on 100 parts by weight of polyether polyol, of a polymer as cell stabilizer. The latter is polymerized by a free radical mechanism in an alcohol and contains incorporated N-vinylpyrrolidone. According to the patent, the entire liquid product obtained by polymerizing the following monomers is used as cell stabilizer:
a) 5 to 60% by weight of N-vinylpyrrolidone,
b) 12.7 to 26.5% by weight of N-vinylpyrrolidone and 14.5 to 38.4% by weight of dibutyl maleate or
c) 10 to 19% by weight of N-vinylpyrrolidone, 20 to 25% by weight of dibutyl maleate and 8 to 16% by weight of vinyl acetate.
The polymerization may optionally be carried out in the presence of a chain transfer agent. The polymerization is initiated with a free radical and is carried out in a liquid polyether polyol with 2 to 8 hydroxyl groups and a molecular weight of 150 to 650. The polymerization proceeds so that at least 30% of the monomers are polymerized. In the patent, it is presumed that the polymerized product is the cell-stabilizing component. However, if the polymer is separated from the polyol, it is less effective and does not regain its effectiveness if it is redissolved in the polyol.
Similar stabilizers are described in U.S. Pat. No. 3,914,188. The copolymer, which is effective as stabilizer, is synthesized by the copolymerization, in a polyether with capped end groups, of compounds of the formula ##STR1## in which n=3, 4 or 5 and the ester of an unsaturated dicarboxylic acid with 4 or 5 carbon atoms. These stabilizing preparations have the advantage that they are largely inert towards isocyanates. Mixtures of these stabilizers with isocyanates can thus be produced, which have a long shelf life.
In the paper "Organic Surfactants for Rigid Urethane and Isocyanurate Foam" by M. J. Skowronski and M. E. Londigran in the Journal of Cellular Plastics 22, 1986, 235 ff., organic stabilizers are described for phenolic resin, isocyanurate and urethane foams. Particularly polymers of fumarate diesters, especially ditridecyl fumarate, are indicated to be useful. The polymerization of these esters was carried out in castor oil, which had been reacted with 36 moles of ethylene oxide in an addition reaction, the polymer having been left in the castor oil.
These known silicon-free stabilizers make it possible to produce rigid polyurethane foams, the surfaces of which can be wetted, painted and glued more satisfactorily. At the same time, the burning behavior of the rigid polyurethane foams is improved. They are, however, not able to fulfill all the required conditions. An important disadvantage of these silicon-free stabilizers as compared to organosilicon stabilizers lies in the reduced solubility of the halocarbon-based blowing agent in the foam system. This reduced solubility is observed especially when polyesterols are used. This is the cause of the inferior thermal insulation effects of the foamed materials, which has been observed in these cases.